Packaged and processed foods, such as cakes, cookies, candies, chocolate, yogurt, and marshmallows, often contain unfamiliar animal ingredients, so may be a special concern for vegetarians due to the likelihood of such additions. Often, prior to purchase or consumption, vegetarians will scrutinize products for animal-derived ingredients. Vegetarians' feelings vary with regard to these ingredients. For example, while some vegetarians may be unaware of animal-derived rennet's role in the production of cheese, and may therefore unknowingly consume the product, other vegetarians may not take issue with its consumption.

Semi-vegetarian diets consist largely of vegetarian foods but may include fish or poultry, or sometimes other meats, on an infrequent basis. Those with diets containing fish or poultry may define meat only as mammalian flesh and may identify with vegetarianism. A pescetarian diet has been described as "fish but no other meat". The common-use association between such diets and vegetarianism has led vegetarian groups such as the Vegetarian Society to state that diets containing these ingredients are not vegetarian, because fish and birds are also animals.

The term "vegetarian" has been in use since 1839 to refer to what was previously described as a "vegetable diet". The word is commonly believed to be a compound of vegetable and the suffix -arian (as in agrarian). According to John Davis, "vegetarian" probably did not directly derive from the Latin word vegetus. The term was popularized with the foundation of the Vegetarian Society in Manchester in 1847, although it may have appeared in print before 1847. The earliest occurrences of the term seem to be related to Alcott House—a school on the north side of Ham Common, London—which was opened in July 1838 by James Pierrepont Greaves. From 1841, it was known as A Concordium, or Industry Harmony College, from which time the institution began to publish its own pamphlet entitled The Healthian, which provides some of the earliest appearances of the term "vegetarian".

The earliest record of vegetarianism comes from Indus Valley Civilization as early as the 7th century BCE, inculcating tolerance towards all living beings. Vegetarianism was also practiced in ancient Greece and the earliest reliable evidence for vegetarian theory and practice in Greece dates from the 6th century BC. The Orphics, a religious movement spreading in Greece at that time, also practiced and promoted vegetarianism. It is unclear whether the Greek religious teacher Pythagoras actually advocated vegetarianism, but later writers presented him as doing so. A fictionalized portrayal of Pythagoras appears in Book XV of Ovid's Metamorphoses, in which he advocates a form of strict vegetarianism. It was through this portrayal that Pythagoras was best known to English-speakers throughout the early modern period and, prior to the coinage of the word "vegetarianism", vegetarians were referred to in English as "Pythagoreans".

Vegetarianism was also practiced about six centuries later in another instance (30 BCE – 50 CE) in the northern Thracian region by the Moesi tribe (who inhabited present-day Serbia and Bulgaria), feeding themselves on honey, milk, and cheese.

In Indian culture, vegetarianism has been closely connected with the attitude of nonviolence towards animals (called ahimsa in India) for millennia and was promoted by religious groups and philosophers. The ancient Indian work of Tirukkural explicitly and unambiguously emphasizes vegetarianism and non-killing. Chapter 26 of the Tirukkural, particularly couplets 251–260, deals exclusively on vegetarianism or veganism. Among the Hellenes, Egyptians, and others, vegetarianism had medical or ritual purification purposes.